The invention relates to a method for determining the depth of field for a specific distance by means of measuring optics and a corresponding device and an arrangement of the device and of recording optics.
The depth of field or depth of focus is a measurement for the extent of the sharp range in the object space of an imaging optical system. The term is of central significance in photography and describes the size of the distance range within which an object appears sufficiently sharply in the image of the camera optics. As a rule, a large depth of focus is achieved using small apertures or lenses with short focal distances.
Two different arrangements can be fundamentally distinguished: The camera obscura which consists of only a single pinhole aperture, and a lens system which also contains such a aperture but additionally (at least) one lens (in front of or behind the aperture) which produces a regular optical image.
Light beams issuing from an object pass through the pinhole aperture onto the image plane (a screen, a film or a camera image sensor). Depending on the diameter of the aperture, these light beams become conical light bodies of greater or lesser thickness. By cutting the image plane with a cone, a circle, a so-called circle of confusion or blurring circle (Z), is produced on the plane. They exist for any dimension of the distances between the object, aperture and image, the size of the circle in the image plane is calculated according to the theorem on intersecting lines. The influence of the pinhole aperture diameter is simply proportional: the larger the hole, the greater the blurring circle. For sharper imaging a smaller hole is required. However, if the hole is made too small, the range of the geometric optics is exited and the wave properties of light come to predominate. The diffraction effects thus arising are all the greater, the smaller the hole. In this way sharpness is reduced. Therefore for a camera obscura there is an optimum hole diameter. Furthermore, in achieving this optimisation, in addition to the imaging properties it is also necessary to consider the circumstance that with a smaller hole diameter the light flux decreases and therefore the illumination times increase.
The structure with an additional lens changes in principle only in that the lens ensures that (in an ideal scenario) when the image plane is at a specific distance from the lens a sharp image is produced, the above-mentioned lack of precision not occurring at this position (and the aperture opening can be substantially increased in the interests of better light yield). Only in the case of object points which lie in front of or behind this sharply imaged position, does this sharpness decrease and, with increasing distance, fall to the value which the aperture would bring about only as a camera obscura.
In geometric optics the only points which are reproduced as sharp image points in the image plane (film, chip sensor) are those which lie on a plane located at the object distance to the lens. All other points located on planes which are closer or further away no longer appear as points in the image plane but rather as small discs, so-called circles of confusion or blurring circles (Z).
Circles of confusion arise because the light bodies falling from the lens onto the image plane are cones; by cutting the image plane with a cone, a circle is produced on the plane.
Points lying close together which do not lie in the object plane are imaged by circles of confusion lying close beside one another and overlapping and mixing together in the edge regions, whereby a blurred image is produced.
It is therefore desirable to be able to know and/or determine the depth of field for a specific distance and optics.
DE 30 35 568 A1 and DE 36 37 742 A1 each disclose photographic cameras which include automatic sharpness adjustments, the optics of which cast an image on a focussing screen.